Network Working Group R. Gilligan
Request for Comments: 2133 Freegate
Category: Informational S. Thomson
Bellcore
J. Bound
Digital
W. Stevens
Consultant
April 1997
Basic Socket Interface Extensions for IPv6
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
The de facto standard application program interface (API) for TCP/IP
applications is the "sockets" interface. Although this API was
developed for Unix in the early 1980s it has also been implemented on
a wide variety of non-Unix systems. TCP/IP applications written
using the sockets API have in the past enjoyed a high degree of
portability and we would like the same portability with IPv6
applications. But changes are required to the sockets API to support
IPv6 and this memo describes these changes. These include a new
socket address structure to carry IPv6 addresses, new address
conversion functions, and some new socket options. These extensions
are designed to provide access to the basic IPv6 features required by
TCP and UDP applications, including multicasting, while introducing a
minimum of change into the system and providing complete
compatibility for existing IPv4 applications. Additional extensions
for advanced IPv6 features (raw sockets and access to the IPv6
extension headers) are defined in another document [5].
Table of Contents
1. Introduction ................................................ 22. Design Considerations ....................................... 32.1. What Needs to be Changed .................................. 32.2. Data Types ................................................ 52.3. Headers ................................................... 52.4. Structures ................................................ 53. Socket Interface ............................................ 53.1. IPv6 Address Family and Protocol Family ................... 53.2. IPv6 Address Structure .................................... 6Gilligan, et. al. Informational [Page 1]

RFC 2133 IPv6 Socket Interface Extensions April 19972. Design Considerations
There are a number of important considerations in designing changes
to this well-worn API:
- The API changes should provide both source and binary
compatibility for programs written to the original API. That is,
existing program binaries should continue to operate when run on
a system supporting the new API. In addition, existing
applications that are re-compiled and run on a system supporting
the new API should continue to operate. Simply put, the API
changes for IPv6 should not break existing programs.
- The changes to the API should be as small as possible in order to
simplify the task of converting existing IPv4 applications to
IPv6.
- Where possible, applications should be able to use this API to
interoperate with both IPv6 and IPv4 hosts. Applications should
not need to know which type of host they are communicating with.
- IPv6 addresses carried in data structures should be 64-bit
aligned. This is necessary in order to obtain optimum
performance on 64-bit machine architectures.
Because of the importance of providing IPv4 compatibility in the API,
these extensions are explicitly designed to operate on machines that
provide complete support for both IPv4 and IPv6. A subset of this
API could probably be designed for operation on systems that support
only IPv6. However, this is not addressed in this memo.
2.1. What Needs to be Changed
The socket interface API consists of a few distinct components:
- Core socket functions.
- Address data structures.
- Name-to-address translation functions.
- Address conversion functions.
The core socket functions -- those functions that deal with such
things as setting up and tearing down TCP connections, and sending
and receiving UDP packets -- were designed to be transport
independent. Where protocol addresses are passed as function
arguments, they are carried via opaque pointers. A protocol-specific
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RFC 2133 IPv6 Socket Interface Extensions April 1997
address data structure is defined for each protocol that the socket
functions support. Applications must cast pointers to these
protocol-specific address structures into pointers to the generic
"sockaddr" address structure when using the socket functions. These
functions need not change for IPv6, but a new IPv6-specific address
data structure is needed.
The "sockaddr_in" structure is the protocol-specific data structure
for IPv4. This data structure actually includes 8-octets of unused
space, and it is tempting to try to use this space to adapt the
sockaddr_in structure to IPv6. Unfortunately, the sockaddr_in
structure is not large enough to hold the 16-octet IPv6 address as
well as the other information (address family and port number) that
is needed. So a new address data structure must be defined for IPv6.
The name-to-address translation functions in the socket interface are
gethostbyname() and gethostbyaddr(). These must be modified to
support IPv6 and the semantics defined must provide 100% backward
compatibility for all existing IPv4 applications, along with IPv6
support for new applications. Additionally, the POSIX 1003.g work in
progress [4] specifies a new hostname-to-address translation function
which is protocol independent. This function can also be used with
IPv6.
The address conversion functions -- inet_ntoa() and inet_addr() --
convert IPv4 addresses between binary and printable form. These
functions are quite specific to 32-bit IPv4 addresses. We have
designed two analogous functions that convert both IPv4 and IPv6
addresses, and carry an address type parameter so that they can be
extended to other protocol families as well.
Finally, a few miscellaneous features are needed to support IPv6.
New interfaces are needed to support the IPv6 flow label, priority,
and hop limit header fields. New socket options are needed to
control the sending and receiving of IPv6 multicast packets.
The socket interface will be enhanced in the future to provide access
to other IPv6 features. These extensions are described in [5].
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RFC 2133 IPv6 Socket Interface Extensions April 19972.2. Data Types
The data types of the structure elements given in this memo are
intended to be examples, not absolute requirements. Whenever
possible, POSIX 1003.1g data types are used: u_intN_t means an
unsigned integer of exactly N bits (e.g., u_int16_t) and u_intNm_t
means an unsigned integer of at least N bits (e.g., u_int32m_t). We
also assume the argument data types from 1003.1g when possible (e.g.,
the final argument to setsockopt() is a size_t value). Whenever
buffer sizes are specified, the POSIX 1003.1 size_t data type is used
(e.g., the two length arguments to getnameinfo()).
2.3. Headers
When function prototypes and structures are shown we show the headers
that must be #included to cause that item to be defined.
2.4. Structures
When structures are described the members shown are the ones that
must appear in an implementation. Additional, nonstandard members
may also be defined by an implementation.
The ordering shown for the members of a structure is the recommended
ordering, given alignment considerations of multibyte members, but an
implementation may order the members differently.
3. Socket Interface
This section specifies the socket interface changes for IPv6.
3.1. IPv6 Address Family and Protocol Family
A new address family name, AF_INET6, is defined in <sys/socket.h>.
The AF_INET6 definition distinguishes between the original
sockaddr_in address data structure, and the new sockaddr_in6 data
structure.
A new protocol family name, PF_INET6, is defined in <sys/socket.h>.
Like most of the other protocol family names, this will usually be
defined to have the same value as the corresponding address family
name:
#define PF_INET6 AF_INET6
The PF_INET6 is used in the first argument to the socket() function
to indicate that an IPv6 socket is being created.
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RFC 2133 IPv6 Socket Interface Extensions April 19973.2. IPv6 Address Structure
A new data structure to hold a single IPv6 address is defined as
follows:
#include <netinet/in.h>
struct in6_addr {
u_int8_t s6_addr[16]; /* IPv6 address */
}
This data structure contains an array of sixteen 8-bit elements,
which make up one 128-bit IPv6 address. The IPv6 address is stored
in network byte order.
3.3. Socket Address Structure for 4.3BSD-Based Systems
In the socket interface, a different protocol-specific data structure
is defined to carry the addresses for each protocol suite. Each
protocol-specific data structure is designed so it can be cast into a
protocol-independent data structure -- the "sockaddr" structure.
Each has a "family" field that overlays the "sa_family" of the
sockaddr data structure. This field identifies the type of the data
structure.
The sockaddr_in structure is the protocol-specific address data
structure for IPv4. It is used to pass addresses between
applications and the system in the socket functions. The following
structure is defined to carry IPv6 addresses:
#include <netinet/in.h>
struct sockaddr_in6 {
u_int16m_t sin6_family; /* AF_INET6 */
u_int16m_t sin6_port; /* transport layer port # */
u_int32m_t sin6_flowinfo; /* IPv6 flow information */
struct in6_addr sin6_addr; /* IPv6 address */
};
This structure is designed to be compatible with the sockaddr data
structure used in the 4.3BSD release.
The sin6_family field identifies this as a sockaddr_in6 structure.
This field overlays the sa_family field when the buffer is cast to a
sockaddr data structure. The value of this field must be AF_INET6.
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RFC 2133 IPv6 Socket Interface Extensions April 1997
The sin6_port field contains the 16-bit UDP or TCP port number. This
field is used in the same way as the sin_port field of the
sockaddr_in structure. The port number is stored in network byte
order.
The sin6_flowinfo field is a 32-bit field that contains two pieces of
information: the 24-bit IPv6 flow label and the 4-bit priority field.
The contents and interpretation of this member is unspecified at this
time.
The sin6_addr field is a single in6_addr structure (defined in the
previous section). This field holds one 128-bit IPv6 address. The
address is stored in network byte order.
The ordering of elements in this structure is specifically designed
so that the sin6_addr field will be aligned on a 64-bit boundary.
This is done for optimum performance on 64-bit architectures.
Notice that the sockaddr_in6 structure will normally be larger than
the generic sockaddr structure. On many existing implementations the
sizeof(struct sockaddr_in) equals sizeof(struct sockaddr), with both
being 16 bytes. Any existing code that makes this assumption needs
to be examined carefully when converting to IPv6.
3.4. Socket Address Structure for 4.4BSD-Based Systems
The 4.4BSD release includes a small, but incompatible change to the
socket interface. The "sa_family" field of the sockaddr data
structure was changed from a 16-bit value to an 8-bit value, and the
space saved used to hold a length field, named "sa_len". The
sockaddr_in6 data structure given in the previous section cannot be
correctly cast into the newer sockaddr data structure. For this
reason, the following alternative IPv6 address data structure is
provided to be used on systems based on 4.4BSD:
#include <netinet/in.h>
#define SIN6_LEN
struct sockaddr_in6 {
u_char sin6_len; /* length of this struct */
u_char sin6_family; /* AF_INET6 */
u_int16m_t sin6_port; /* transport layer port # */
u_int32m_t sin6_flowinfo; /* IPv6 flow information */
struct in6_addr sin6_addr; /* IPv6 address */
};
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RFC 2133 IPv6 Socket Interface Extensions April 1997
The only differences between this data structure and the 4.3BSD
variant are the inclusion of the length field, and the change of the
family field to a 8-bit data type. The definitions of all the other
fields are identical to the structure defined in the previous
section.
Systems that provide this version of the sockaddr_in6 data structure
must also declare SIN6_LEN as a result of including the
<netinet/in.h> header. This macro allows applications to determine
whether they are being built on a system that supports the 4.3BSD or
4.4BSD variants of the data structure.
3.5. The Socket Functions
Applications call the socket() function to create a socket descriptor
that represents a communication endpoint. The arguments to the
socket() function tell the system which protocol to use, and what
format address structure will be used in subsequent functions. For
example, to create an IPv4/TCP socket, applications make the call:
s = socket(PF_INET, SOCK_STREAM, 0);
To create an IPv4/UDP socket, applications make the call:
s = socket(PF_INET, SOCK_DGRAM, 0);
Applications may create IPv6/TCP and IPv6/UDP sockets by simply using
the constant PF_INET6 instead of PF_INET in the first argument. For
example, to create an IPv6/TCP socket, applications make the call:
s = socket(PF_INET6, SOCK_STREAM, 0);
To create an IPv6/UDP socket, applications make the call:
s = socket(PF_INET6, SOCK_DGRAM, 0);
Once the application has created a PF_INET6 socket, it must use the
sockaddr_in6 address structure when passing addresses in to the
system. The functions that the application uses to pass addresses
into the system are:
bind()
connect()
sendmsg()
sendto()
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RFC 2133 IPv6 Socket Interface Extensions April 1997
The system will use the sockaddr_in6 address structure to return
addresses to applications that are using PF_INET6 sockets. The
functions that return an address from the system to an application
are:
accept()
recvfrom()
recvmsg()
getpeername()
getsockname()
No changes to the syntax of the socket functions are needed to
support IPv6, since all of the "address carrying" functions use an
opaque address pointer, and carry an address length as a function
argument.
3.6. Compatibility with IPv4 Applications
In order to support the large base of applications using the original
API, system implementations must provide complete source and binary
compatibility with the original API. This means that systems must
continue to support PF_INET sockets and the sockaddr_in address
structure. Applications must be able to create IPv4/TCP and IPv4/UDP
sockets using the PF_INET constant in the socket() function, as
described in the previous section. Applications should be able to
hold a combination of IPv4/TCP, IPv4/UDP, IPv6/TCP and IPv6/UDP
sockets simultaneously within the same process.
Applications using the original API should continue to operate as
they did on systems supporting only IPv4. That is, they should
continue to interoperate with IPv4 nodes.
3.7. Compatibility with IPv4 Nodes
The API also provides a different type of compatibility: the ability
for IPv6 applications to interoperate with IPv4 applications. This
feature uses the IPv4-mapped IPv6 address format defined in the IPv6
addressing architecture specification [2]. This address format
allows the IPv4 address of an IPv4 node to be represented as an IPv6
address. The IPv4 address is encoded into the low-order 32 bits of
the IPv6 address, and the high-order 96 bits hold the fixed prefix
0:0:0:0:0:FFFF. IPv4-mapped addresses are written as follows:
::FFFF:<IPv4-address>
These addresses are often generated automatically by the
gethostbyname() function when the specified host has only IPv4
addresses (as described in Section 6.1).
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RFC 2133 IPv6 Socket Interface Extensions April 1997
Applications may use PF_INET6 sockets to open TCP connections to IPv4
nodes, or send UDP packets to IPv4 nodes, by simply encoding the
destination's IPv4 address as an IPv4-mapped IPv6 address, and
passing that address, within a sockaddr_in6 structure, in the
connect() or sendto() call. When applications use PF_INET6 sockets
to accept TCP connections from IPv4 nodes, or receive UDP packets
from IPv4 nodes, the system returns the peer's address to the
application in the accept(), recvfrom(), or getpeername() call using
a sockaddr_in6 structure encoded this way.
Few applications will likely need to know which type of node they are
interoperating with. However, for those applications that do need to
know, the IN6_IS_ADDR_V4MAPPED() macro, defined in Section 6.6, is
provided.
3.8. IPv6 Wildcard Address
While the bind() function allows applications to select the source IP
address of UDP packets and TCP connections, applications often want
the system to select the source address for them. With IPv4, one
specifies the address as the symbolic constant INADDR_ANY (called the
"wildcard" address) in the bind() call, or simply omits the bind()
entirely.
Since the IPv6 address type is a structure (struct in6_addr), a
symbolic constant can be used to initialize an IPv6 address variable,
but cannot be used in an assignment. Therefore systems provide the
IPv6 wildcard address in two forms.
The first version is a global variable named "in6addr_any" that is an
in6_addr structure. The extern declaration for this variable is
defined in <netinet/in.h>:
extern const struct in6_addr in6addr_any;
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RFC 2133 IPv6 Socket Interface Extensions April 1997
Applications use in6addr_any similarly to the way they use INADDR_ANY
in IPv4. For example, to bind a socket to port number 23, but let
the system select the source address, an application could use the
following code:
struct sockaddr_in6 sin6;
. . .
sin6.sin6_family = AF_INET6;
sin6.sin6_flowinfo = 0;
sin6.sin6_port = htons(23);
sin6.sin6_addr = in6addr_any; /* structure assignment */
. . .
if (bind(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
. . .
The other version is a symbolic constant named IN6ADDR_ANY_INIT and
is defined in <netinet/in.h>. This constant can be used to
initialize an in6_addr structure:
struct in6_addr anyaddr = IN6ADDR_ANY_INIT;
Note that this constant can be used ONLY at declaration time. It can
not be used to assign a previously declared in6_addr structure. For
example, the following code will not work:
/* This is the WRONG way to assign an unspecified address */
struct sockaddr_in6 sin6;
. . .
sin6.sin6_addr = IN6ADDR_ANY_INIT; /* will NOT compile */
Be aware that the IPv4 INADDR_xxx constants are all defined in host
byte order but the IPv6 IN6ADDR_xxx constants and the IPv6
in6addr_xxx externals are defined in network byte order.
3.9. IPv6 Loopback Address
Applications may need to send UDP packets to, or originate TCP
connections to, services residing on the local node. In IPv4, they
can do this by using the constant IPv4 address INADDR_LOOPBACK in
their connect(), sendto(), or sendmsg() call.
IPv6 also provides a loopback address to contact local TCP and UDP
services. Like the unspecified address, the IPv6 loopback address is
provided in two forms -- a global variable and a symbolic constant.
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RFC 2133 IPv6 Socket Interface Extensions April 1997
The global variable is an in6_addr structure named
"in6addr_loopback." The extern declaration for this variable is
defined in <netinet/in.h>:
extern const struct in6_addr in6addr_loopback;
Applications use in6addr_loopback as they would use INADDR_LOOPBACK
in IPv4 applications (but beware of the byte ordering difference
mentioned at the end of the previous section). For example, to open
a TCP connection to the local telnet server, an application could use
the following code:
struct sockaddr_in6 sin6;
. . .
sin6.sin6_family = AF_INET6;
sin6.sin6_flowinfo = 0;
sin6.sin6_port = htons(23);
sin6.sin6_addr = in6addr_loopback; /* structure assignment */
. . .
if (connect(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
. . .
The symbolic constant is named IN6ADDR_LOOPBACK_INIT and is defined
in <netinet/in.h>. It can be used at declaration time ONLY; for
example:
struct in6_addr loopbackaddr = IN6ADDR_LOOPBACK_INIT;
Like IN6ADDR_ANY_INIT, this constant cannot be used in an assignment
to a previously declared IPv6 address variable.
4. Interface Identification
This API uses an interface index (a small positive integer) to
identify the local interface on which a multicast group is joined
(Section 5.3). Additionally, the advanced API [5] uses these same
interface indexes to identify the interface on which a datagram is
received, or to specify the interface on which a datagram is to be
sent.
Interfaces are normally known by names such as "le0", "sl1", "ppp2",
and the like. On Berkeley-derived implementations, when an interface
is made known to the system, the kernel assigns a unique positive
integer value (called the interface index) to that interface. These
are small positive integers that start at 1. (Note that 0 is never
used for an interface index.) There may be gaps so that there is no
current interface for a particular positive interface index.
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RFC 2133 IPv6 Socket Interface Extensions April 1997
This API defines two functions that map between an interface name and
index, a third function that returns all the interface names and
indexes, and a fourth function to return the dynamic memory allocated
by the previous function. How these functions are implemented is
left up to the implementation. 4.4BSD implementations can implement
these functions using the existing sysctl() function with the
NET_RT_LIST command. Other implementations may wish to use ioctl()
for this purpose.
4.1. Name-to-Index
The first function maps an interface name into its corresponding
index.
#include <net/if.h>
unsigned int if_nametoindex(const char *ifname);
If the specified interface does not exist, the return value is 0.
4.2. Index-to-Name
The second function maps an interface index into its corresponding
name.
#include <net/if.h>
char *if_indextoname(unsigned int ifindex, char *ifname);
The ifname argument must point to a buffer of at least IFNAMSIZ bytes
into which the interface name corresponding to the specified index is
returned. (IFNAMSIZ is also defined in <net/if.h> and its value
includes a terminating null byte at the end of the interface name.)
This pointer is also the return value of the function. If there is
no interface corresponding to the specified index, NULL is returned.
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RFC 2133 IPv6 Socket Interface Extensions April 19974.3. Return All Interface Names and Indexes
The final function returns an array of if_nameindex structures, one
structure per interface.
#include <net/if.h>
struct if_nameindex {
unsigned int if_index; /* 1, 2, ... */
char *if_name; /* null terminated name: "le0", ... */
};
struct if_nameindex *if_nameindex(void);
The end of the array of structures is indicated by a structure with
an if_index of 0 and an if_name of NULL. The function returns a NULL
pointer upon an error.
The memory used for this array of structures along with the interface
names pointed to by the if_name members is obtained dynamically.
This memory is freed by the next function.
4.4. Free Memory
The following function frees the dynamic memory that was allocated by
if_nameindex().
#include <net/if.h>
void if_freenameindex(struct if_nameindex *ptr);
The argument to this function must be a pointer that was returned by
if_nameindex().
5. Socket Options
A number of new socket options are defined for IPv6. All of these
new options are at the IPPROTO_IPV6 level. That is, the "level"
parameter in the getsockopt() and setsockopt() calls is IPPROTO_IPV6
when using these options. The constant name prefix IPV6_ is used in
all of the new socket options. This serves to clearly identify these
options as applying to IPv6.
The declaration for IPPROTO_IPV6, the new IPv6 socket options, and
related constants defined in this section are obtained by including
the header <netinet/in.h>.
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RFC 2133 IPv6 Socket Interface Extensions April 19975.1. Changing Socket Type
Unix allows open sockets to be passed between processes via the
exec() call and other means. It is a relatively common application
practice to pass open sockets across exec() calls. Thus it is
possible for an application using the original API to pass an open
PF_INET socket to an application that is expecting to receive a
PF_INET6 socket. Similarly, it is possible for an application using
the extended API to pass an open PF_INET6 socket to an application
using the original API, which would be equipped only to deal with
PF_INET sockets. Either of these cases could cause problems, because
the application that is passed the open socket might not know how to
decode the address structures returned in subsequent socket
functions.
To remedy this problem, a new setsockopt() option is defined that
allows an application to "convert" a PF_INET6 socket into a PF_INET
socket and vice versa.
An IPv6 application that is passed an open socket from an unknown
process may use the IPV6_ADDRFORM setsockopt() option to "convert"
the socket to PF_INET6. Once that has been done, the system will
return sockaddr_in6 address structures in subsequent socket
functions.
An IPv6 application that is about to pass an open PF_INET6 socket to
a program that is not be IPv6 capable can "downgrade" the socket to
PF_INET before calling exec(). After that, the system will return
sockaddr_in address structures to the application that was exec()'ed.
Be aware that you cannot downgrade an IPv6 socket to an IPv4 socket
unless all nonwildcard addresses already associated with the IPv6
socket are IPv4-mapped IPv6 addresses.
The IPV6_ADDRFORM option is valid at both the IPPROTO_IP and
IPPROTO_IPV6 levels. The only valid option values are PF_INET6 and
PF_INET. For example, to convert a PF_INET6 socket to PF_INET, a
program would call:
int addrform = PF_INET;
if (setsockopt(s, IPPROTO_IPV6, IPV6_ADDRFORM,
(char *) &addrform, sizeof(addrform)) == -1)
perror("setsockopt IPV6_ADDRFORM");
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RFC 2133 IPv6 Socket Interface Extensions April 1997
An application may use IPV6_ADDRFORM with getsockopt() to learn
whether an open socket is a PF_INET of PF_INET6 socket. For example:
int addrform;
size_t len = sizeof(addrform);
if (getsockopt(s, IPPROTO_IPV6, IPV6_ADDRFORM,
(char *) &addrform, &len) == -1)
perror("getsockopt IPV6_ADDRFORM");
else if (addrform == PF_INET)
printf("This is an IPv4 socket.\n");
else if (addrform == PF_INET6)
printf("This is an IPv6 socket.\n");
else
printf("This system is broken.\n");
5.2. Unicast Hop Limit
A new setsockopt() option controls the hop limit used in outgoing
unicast IPv6 packets. The name of this option is IPV6_UNICAST_HOPS,
and it is used at the IPPROTO_IPV6 layer. The following example
illustrates how it is used:
int hoplimit = 10;
if (setsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS,
(char *) &hoplimit, sizeof(hoplimit)) == -1)
perror("setsockopt IPV6_UNICAST_HOPS");
When the IPV6_UNICAST_HOPS option is set with setsockopt(), the
option value given is used as the hop limit for all subsequent
unicast packets sent via that socket. If the option is not set, the
system selects a default value. The integer hop limit value (called
x) is interpreted as follows:
x < -1: return an error of EINVAL
x == -1: use kernel default
0 <= x <= 255: use x
x >= 256: return an error of EINVAL
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RFC 2133 IPv6 Socket Interface Extensions April 1997
The IPV6_UNICAST_HOPS option may be used with getsockopt() to
determine the hop limit value that the system will use for subsequent
unicast packets sent via that socket. For example:
int hoplimit;
size_t len = sizeof(hoplimit);
if (getsockopt(s, IPPROTO_IPV6, IPV6_UNICAST_HOPS,
(char *) &hoplimit, &len) == -1)
perror("getsockopt IPV6_UNICAST_HOPS");
else
printf("Using %d for hop limit.\n", hoplimit);
5.3. Sending and Receiving Multicast Packets
IPv6 applications may send UDP multicast packets by simply specifying
an IPv6 multicast address in the address argument of the sendto()
function.
Three socket options at the IPPROTO_IPV6 layer control some of the
parameters for sending multicast packets. Setting these options is
not required: applications may send multicast packets without using
these options. The setsockopt() options for controlling the sending
of multicast packets are summarized below:
IPV6_MULTICAST_IF
Set the interface to use for outgoing multicast packets. The
argument is the index of the interface to use.
Argument type: unsigned int
IPV6_MULTICAST_HOPS
Set the hop limit to use for outgoing multicast packets.
(Note a separate option - IPV6_UNICAST_HOPS - is provided to
set the hop limit to use for outgoing unicast packets.) The
interpretation of the argument is the same as for the
IPV6_UNICAST_HOPS option:
x < -1: return an error of EINVAL
x == -1: use kernel default
0 <= x <= 255: use x
x >= 256: return an error of EINVAL
Argument type: int
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RFC 2133 IPv6 Socket Interface Extensions April 1997
IPV6_MULTICAST_LOOP
Controls whether outgoing multicast packets sent should be
delivered back to the local application. A toggle. If the
option is set to 1, multicast packets are looped back. If it
is set to 0, they are not.
Argument type: unsigned int
The reception of multicast packets is controlled by the two
setsockopt() options summarized below:
IPV6_ADD_MEMBERSHIP
Join a multicast group on a specified local interface. If
the interface index is specified as 0, the kernel chooses the
local interface. For example, some kernels look up the
multicast group in the normal IPv6 routing table and using
the resulting interface.
Argument type: struct ipv6_mreq
IPV6_DROP_MEMBERSHIP
Leave a multicast group on a specified interface.
Argument type: struct ipv6_mreq
The argument type of both of these options is the ipv6_mreq
structure, defined as:
#include <netinet/in.h>
struct ipv6_mreq {
struct in6_addr ipv6mr_multiaddr; /* IPv6 multicast addr */
unsigned int ipv6mr_interface; /* interface index */
};
Note that to receive multicast datagrams a process must join the
multicast group and bind the UDP port to which datagrams will be
sent. Some processes also bind the multicast group address to the
socket, in addition to the port, to prevent other datagrams destined
to that same port from being delivered to the socket.
Gilligan, et. al. Informational [Page 18]

RFC 2133 IPv6 Socket Interface Extensions April 19976. Library Functions
New library functions are needed to perform a variety of operations
with IPv6 addresses. Functions are needed to lookup IPv6 addresses
in the Domain Name System (DNS). Both forward lookup (hostname-to-
address translation) and reverse lookup (address-to-hostname
translation) need to be supported. Functions are also needed to
convert IPv6 addresses between their binary and textual form.
6.1. Hostname-to-Address Translation
The commonly used function gethostbyname() remains unchanged as does
the hostent structure to which it returns a pointer. Existing
applications that call this function continue to receive only IPv4
addresses that are the result of a query in the DNS for A records.
(We assume the DNS is being used; some environments may be using a
hosts file or some other name resolution system, either of which may
impede renumbering. We also assume that the RES_USE_INET6 resolver
option is not set, which we describe in more detail shortly.)
Two new changes are made to support IPv6 addresses. First, the
following function is new:
#include <sys/socket.h>
#include <netdb.h>
struct hostent *gethostbyname2(const char *name, int af);
The af argument specifies the address family. The default operation
of this function is simple:
- If the af argument is AF_INET, then a query is made for A
records. If successful, IPv4 addresses are returned and the
h_length member of the hostent structure will be 4, else the
function returns a NULL pointer.
- If the af argument is AF_INET6, then a query is made for AAAA
records. If successful, IPv6 addresses are returned and the
h_length member of the hostent structure will be 16, else the
function returns a NULL pointer.
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RFC 2133 IPv6 Socket Interface Extensions April 1997
The second change, that provides additional functionality, is a new
resolver option RES_USE_INET6, which is defined as a result of
including the <resolv.h> header. (This option is provided starting
with the BIND 4.9.4 release.) There are three ways to set this
option.
- The first way is
res_init();
_res.options |= RES_USE_INET6;
and then call either gethostbyname() or gethostbyname2(). This
option then affects only the process that is calling the
resolver.
- The second way to set this option is to set the environment
variable RES_OPTIONS, as in RES_OPTIONS=inet6. (This example is
for the Bourne and Korn shells.) This method affects any
processes that see this environment variable.
- The third way is to set this option in the resolver configuration
file (normally /etc/resolv.conf) and the option then affects all
applications on the host. This final method should not be done
until all applications on the host are capable of dealing with
IPv6 addresses.
There is no priority among these three methods. When the
RES_USE_INET6 option is set, two changes occur:
- gethostbyname(host) first calls gethostbyname2(host, AF_INET6)
looking for AAAA records, and if this fails it then calls
gethostbyname2(host, AF_INET) looking for A records.
- gethostbyname2(host, AF_INET) always returns IPv4-mapped IPv6
addresses with the h_length member of the hostent structure set
to 16.
An application must not enable the RES_USE_INET6 option until it is
prepared to deal with 16-byte addresses in the returned hostent
structure.
Gilligan, et. al. Informational [Page 20]

RFC 2133 IPv6 Socket Interface Extensions April 19976.2. Address To Hostname Translation
The existing gethostbyaddr() function already requires an address
family argument and can therefore work with IPv6 addresses:
#include <sys/socket.h>
#include <netdb.h>
struct hostent *gethostbyaddr(const char *src, int len, int af);
One possible source of confusion is the handling of IPv4-mapped IPv6
addresses and IPv4-compatible IPv6 addresses. This is addressed in
[6] and involves the following logic:
1. If af is AF_INET6, and if len equals 16, and if the IPv6 address
is an IPv4-mapped IPv6 address or an IPv4-compatible IPv6
address, then skip over the first 12 bytes of the IPv6 address,
set af to AF_INET, and set len to 4.
2. If af is AF_INET, then query for a PTR record in the in-
addr.arpa domain.
3. If af is AF_INET6, then query for a PTR record in the ip6.int
domain.
4. If the function is returning success, and if af equals AF_INET,
and if the RES_USE_INET6 option was set, then the single address
that is returned in the hostent structure (a copy of the first
argument to the function) is returned as an IPv4-mapped IPv6
address and the h_length member is set to 16.
All four steps listed are performed, in order. The same caveats
regarding a thread-safe version of gethostbyname() that were made at
the end of the previous section apply here as well.
6.3. Protocol-Independent Hostname and Service Name Translation
Hostname-to-address translation is done in a protocol-independent
fashion using the getaddrinfo() function that is taken from the
Institute of Electrical and Electronic Engineers (IEEE) POSIX 1003.1g
(Protocol Independent Interfaces) work in progress specification [4].
The official specification for this function will be the final POSIX
standard. We are providing this independent description of the
function because POSIX standards are not freely available (as are
IETF documents). Should there be any discrepancies between this
description and the POSIX description, the POSIX description takes
precedence.
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RFC 2133 IPv6 Socket Interface Extensions April 1997
#include <sys/socket.h>
#include <netdb.h>
int getaddrinfo(const char *hostname, const char *servname,
const struct addrinfo *hints,
struct addrinfo **res);
The addrinfo structure is defined as:
#include <sys/socket.h>
#include <netdb.h>
struct addrinfo {
int ai_flags; /* AI_PASSIVE, AI_CANONNAME */
int ai_family; /* PF_xxx */
int ai_socktype; /* SOCK_xxx */
int ai_protocol; /* 0 or IPPROTO_xxx for IPv4 and IPv6 */
size_t ai_addrlen; /* length of ai_addr */
char *ai_canonname; /* canonical name for hostname */
struct sockaddr *ai_addr; /* binary address */
struct addrinfo *ai_next; /* next structure in linked list */
};
The return value from the function is 0 upon success or a nonzero
error code. The following names are the nonzero error codes from
getaddrinfo(), and are defined in <netdb.h>:
EAI_ADDRFAMILY address family for hostname not supported
EAI_AGAIN temporary failure in name resolution
EAI_BADFLAGS invalid value for ai_flags
EAI_FAIL non-recoverable failure in name resolution
EAI_FAMILY ai_family not supported
EAI_MEMORY memory allocation failure
EAI_NODATA no address associated with hostname
EAI_NONAME hostname nor servname provided, or not known
EAI_SERVICE servname not supported for ai_socktype
EAI_SOCKTYPE ai_socktype not supported
EAI_SYSTEM system error returned in errno
The hostname and servname arguments are pointers to null-terminated
strings or NULL. One or both of these two arguments must be a non-
NULL pointer. In the normal client scenario, both the hostname and
servname are specified. In the normal server scenario, only the
servname is specified. A non-NULL hostname string can be either a
host name or a numeric host address string (i.e., a dotted-decimal
IPv4 address or an IPv6 hex address). A non-NULL servname string can
be either a service name or a decimal port number.
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RFC 2133 IPv6 Socket Interface Extensions April 1997
The caller can optionally pass an addrinfo structure, pointed to by
the third argument, to provide hints concerning the type of socket
that the caller supports. In this hints structure all members other
than ai_flags, ai_family, ai_socktype, and ai_protocol must be zero
or a NULL pointer. A value of PF_UNSPEC for ai_family means the
caller will accept any protocol family. A value of 0 for ai_socktype
means the caller will accept any socket type. A value of 0 for
ai_protocol means the caller will accept any protocol. For example,
if the caller handles only TCP and not UDP, then the ai_socktype
member of the hints structure should be set to SOCK_STREAM when
getaddrinfo() is called. If the caller handles only IPv4 and not
IPv6, then the ai_family member of the hints structure should be set
to PF_INET when getaddrinfo() is called. If the third argument to
getaddrinfo() is a NULL pointer, this is the same as if the caller
had filled in an addrinfo structure initialized to zero with
ai_family set to PF_UNSPEC.
Upon successful return a pointer to a linked list of one or more
addrinfo structures is returned through the final argument. The
caller can process each addrinfo structure in this list by following
the ai_next pointer, until a NULL pointer is encountered. In each
returned addrinfo structure the three members ai_family, ai_socktype,
and ai_protocol are the corresponding arguments for a call to the
socket() function. In each addrinfo structure the ai_addr member
points to a filled-in socket address structure whose length is
specified by the ai_addrlen member.
If the AI_PASSIVE bit is set in the ai_flags member of the hints
structure, then the caller plans to use the returned socket address
structure in a call to bind(). In this case, if the hostname
argument is a NULL pointer, then the IP address portion of the socket
address structure will be set to INADDR_ANY for an IPv4 address or
IN6ADDR_ANY_INIT for an IPv6 address.
If the AI_PASSIVE bit is not set in the ai_flags member of the hints
structure, then the returned socket address structure will be ready
for a call to connect() (for a connection-oriented protocol) or
either connect(), sendto(), or sendmsg() (for a connectionless
protocol). In this case, if the hostname argument is a NULL pointer,
then the IP address portion of the socket address structure will be
set to the loopback address.
If the AI_CANONNAME bit is set in the ai_flags member of the hints
structure, then upon successful return the ai_canonname member of the
first addrinfo structure in the linked list will point to a null-
terminated string containing the canonical name of the specified
hostname.
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RFC 2133 IPv6 Socket Interface Extensions April 1997
All of the information returned by getaddrinfo() is dynamically
allocated: the addrinfo structures, and the socket address structures
and canonical host name strings pointed to by the addrinfo
structures. To return this information to the system the function
freeaddrinfo() is called:
#include <sys/socket.h>
#include <netdb.h>
void freeaddrinfo(struct addrinfo *ai);
The addrinfo structure pointed to by the ai argument is freed, along
with any dynamic storage pointed to by the structure. This operation
is repeated until a NULL ai_next pointer is encountered.
To aid applications in printing error messages based on the EAI_xxx
codes returned by getaddrinfo(), the following function is defined.
#include <sys/socket.h>
#include <netdb.h>
char *gai_strerror(int ecode);
The argument is one of the EAI_xxx values defined earlier and the
eturn value points to a string describing the error. If the argument
is not one of the EAI_xxx values, the function still returns a
pointer to a string whose contents indicate an unknown error.
6.4. Socket Address Structure to Hostname and Service Name
The POSIX 1003.1g specification includes no function to perform the
reverse conversion from getaddrinfo(): to look up a hostname and
service name, given the binary address and port. Therefore, we
define the following function:
#include <sys/socket.h>
#include <netdb.h>
int getnameinfo(const struct sockaddr *sa, size_t salen,
char *host, size_t hostlen,
char *serv, size_t servlen,
int flags);
This function looks up an IP address and port number provided by the
caller in the DNS and system-specific database, and returns text
strings for both in buffers provided by the caller. The function
indicates successful completion by a zero return value; a non-zero
return value indicates failure.
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RFC 2133 IPv6 Socket Interface Extensions April 1997
The first argument, sa, points to either a sockaddr_in structure (for
IPv4) or a sockaddr_in6 structure (for IPv6) that holds the IP
address and port number. The salen argument gives the length of the
sockaddr_in or sockaddr_in6 structure.
The function returns the hostname associated with the IP address in
the buffer pointed to by the host argument. The caller provides the
size of this buffer via the hostlen argument. The service name
associated with the port number is returned in the buffer pointed to
by serv, and the servlen argument gives the length of this buffer.
The caller specifies not to return either string by providing a zero
value for the hostlen or servlen arguments. Otherwise, the caller
must provide buffers large enough to hold the hostname and the
service name, including the terminating null characters.
Unfortunately most systems do not provide constants that specify the
maximum size of either a fully-qualified domain name or a service
name. Therefore to aid the application in allocating buffers for
these two returned strings the following constants are defined in
<netdb.h>:
#define NI_MAXHOST 1025
#define NI_MAXSERV 32
The first value is actually defined as the constant MAXDNAME in
recent versions of BIND's <arpa/nameser.h> header (older versions of
BIND define this constant to be 256) and the second is a guess based
on the services listed in the current Assigned Numbers RFC.
The final argument is a flag that changes the default actions of this
function. By default the fully-qualified domain name (FQDN) for the
host is looked up in the DNS and returned. If the flag bit NI_NOFQDN
is set, only the hostname portion of the FQDN is returned for local
hosts.
If the flag bit NI_NUMERICHOST is set, or if the host's name cannot
be located in the DNS, the numeric form of the host's address is
returned instead of its name (e.g., by calling inet_ntop() instead of
gethostbyaddr()). If the flag bit NI_NAMEREQD is set, an error is
returned if the host's name cannot be located in the DNS.
If the flag bit NI_NUMERICSERV is set, the numeric form of the
service address is returned (e.g., its port number) instead of its
name. The two NI_NUMERICxxx flags are required to support the "-n"
flag that many commands provide.
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RFC 2133 IPv6 Socket Interface Extensions April 1997
A fifth flag bit, NI_DGRAM, specifies that the service is a datagram
service, and causes getservbyport() to be called with a second
argument of "udp" instead of its default of "tcp". This is required
for the few ports (512-514) that have different services for UDP and
TCP.
These NI_xxx flags are defined in <netdb.h> along with the AI_xxx
flags already defined for getaddrinfo().
6.5. Address Conversion Functions
The two functions inet_addr() and inet_ntoa() convert an IPv4 address
between binary and text form. IPv6 applications need similar
functions. The following two functions convert both IPv6 and IPv4
addresses:
#include <sys/socket.h>
#include <arpa/inet.h>
int inet_pton(int af, const char *src, void *dst);
const char *inet_ntop(int af, const void *src,
char *dst, size_t size);
The inet_pton() function converts an address in its standard text
presentation form into its numeric binary form. The af argument
specifies the family of the address. Currently the AF_INET and
AF_INET6 address families are supported. The src argument points to
the string being passed in. The dst argument points to a buffer into
which the function stores the numeric address. The address is
returned in network byte order. Inet_pton() returns 1 if the
conversion succeeds, 0 if the input is not a valid IPv4 dotted-
decimal string or a valid IPv6 address string, or -1 with errno set
to EAFNOSUPPORT if the af argument is unknown. The calling
application must ensure that the buffer referred to by dst is large
enough to hold the numeric address (e.g., 4 bytes for AF_INET or 16
bytes for AF_INET6).
If the af argument is AF_INET, the function accepts a string in the
standard IPv4 dotted-decimal form:
ddd.ddd.ddd.ddd
where ddd is a one to three digit decimal number between 0 and 255.
Note that many implementations of the existing inet_addr() and
inet_aton() functions accept nonstandard input: octal numbers,
hexadecimal numbers, and fewer than four numbers. inet_pton() does
not accept these formats.
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RFC 2133 IPv6 Socket Interface Extensions April 1997
If the af argument is AF_INET6, then the function accepts a string in
one of the standard IPv6 text forms defined in Section 2.2 of the
addressing architecture specification [2].
The inet_ntop() function converts a numeric address into a text
string suitable for presentation. The af argument specifies the
family of the address. This can be AF_INET or AF_INET6. The src
argument points to a buffer holding an IPv4 address if the af
argument is AF_INET, or an IPv6 address if the af argument is
AF_INET6. The dst argument points to a buffer where the function
will store the resulting text string. The size argument specifies
the size of this buffer. The application must specify a non-NULL dst
argument. For IPv6 addresses, the buffer must be at least 46-octets.
For IPv4 addresses, the buffer must be at least 16-octets. In order
to allow applications to easily declare buffers of the proper size to
store IPv4 and IPv6 addresses in string form, the following two
constants are defined in <netinet/in.h>:
#define INET_ADDRSTRLEN 16
#define INET6_ADDRSTRLEN 46
The inet_ntop() function returns a pointer to the buffer containing
the text string if the conversion succeeds, and NULL otherwise. Upon
failure, errno is set to EAFNOSUPPORT if the af argument is invalid
or ENOSPC if the size of the result buffer is inadequate.
6.6. Address Testing Macros
The following macros can be used to test for special IPv6 addresses.
#include <netinet/in.h>
int IN6_IS_ADDR_UNSPECIFIED (const struct in6_addr *);
int IN6_IS_ADDR_LOOPBACK (const struct in6_addr *);
int IN6_IS_ADDR_MULTICAST (const struct in6_addr *);
int IN6_IS_ADDR_LINKLOCAL (const struct in6_addr *);
int IN6_IS_ADDR_SITELOCAL (const struct in6_addr *);
int IN6_IS_ADDR_V4MAPPED (const struct in6_addr *);
int IN6_IS_ADDR_V4COMPAT (const struct in6_addr *);
int IN6_IS_ADDR_MC_NODELOCAL(const struct in6_addr *);
int IN6_IS_ADDR_MC_LINKLOCAL(const struct in6_addr *);
int IN6_IS_ADDR_MC_SITELOCAL(const struct in6_addr *);
int IN6_IS_ADDR_MC_ORGLOCAL (const struct in6_addr *);
int IN6_IS_ADDR_MC_GLOBAL (const struct in6_addr *);
Gilligan, et. al. Informational [Page 28]